Iterative multi-stage detection technique for a diversity receiver having multiple antenna elements

ABSTRACT

An iterative multistage detection system and method for orthogonally multiplexing K channels onto a signal processing chain using N orthogonal sequences of length N. The K channels include a first set of N channels and a second set of M channels (the M channels being separate and distinct from the N channels), where K=N+M. In a first iteration, interference from the first set of N channels imparted on the second set of M channels is removed from the multiplexed signal, thereby enabling the symbol values associated with the second set of M channels to be reliably estimated. In a second iteration, interference from the second set of M channels imparted on the first set of N channels is removed from the first set of N-channels, thereby enabling the symbol values associated with the first set of N channels to be reliably estimated.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Application No. 60/407,524 entitled ITERATIVE MULTI-STAGEDETECTION TECHNIQUE FOR DIVERSITY RECEIVER HAVING MULTIPLE ANTENNAELEMENTS, filed Aug. 28, 2002, which is incorporated herein by referencein its entirety

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an antenna diversity receiver for radiocommunication systems, and more particularly to a multi-channeldetection process implemented in a receiver receiving signals overmultiple channels.

2. Background Information

It has recently been proposed that both the performance and capacity ofexisting wireless systems could be improved through the use of so-called“smart” antenna techniques. In particular, it has been suggested thatsuch techniques, coupled with space-time signal processing, could beutilized both to combat the deleterious effects of multipath fading of adesired incoming signal and to suppress interfering signals. In this wayboth performance and capacity of digital wireless systems in existenceor being deployed (e.g., CDMA-based systems, TDMA-based systems, WLANsystems, and OFDM-based systems such as IEEE 802.11a/g) may be improved.

It is anticipated that smart antenna techniques will be increasinglyutilized both in connection with deployment of base stationinfrastructure and mobile subscriber units (e.g, handsets) in cellularsystems in order to address the increasing demands being placed uponsuch systems. These demands are arising in part from the shift underwayfrom current voice-based services to next-generation wireless multimediaservices and the accompanying blurring of distinctions among voice,video and data modes of transmission. Subscriber units utilized in suchnext-generation systems will likely be required to demonstrate highervoice quality relative to existing cellular mobile radio standards aswell as to provide high-speed data services (e.g., as high as 10Mbits/s). Achieving high speed and high quality of service, however, iscomplicated because it is desirable for mobile subscriber units to besmall and lightweight, and to be capable of reliably operating in avariety of environments (e.g., cellular/microcellular/picocellular,urban/suburban/rural and indoor/outdoor). Moreover, in addition tooffering higher-quality communication and coverage, next-generationsystems are desired to more efficiently use available bandwidth and tobe priced affordably to ensure widespread market adoption.

In many wireless systems, three principal factors tend to account forthe bulk of performance and capacity degradation: multipath fading,delay spread between received multipath signal components, andco-channel interference (CCI). As is known, multipath fading is causedby the multiple paths which may be traversed by a transmitted signal enroute to a receive antenna. The signals from these paths add togetherwith different phases, resulting in a received signal amplitude andphase that vary with antenna location, direction and polarization, aswell as with time (as a result of movement through the environment).Increasing the quality or reducing the effective error rate in order toobviate the effects of multipath fading has proven to be extremelydifficult. Although it would be theoretically possible to reduce theeffects of multipath fading through use of higher transmit power oradditional bandwidth, these approaches are often inconsistent with therequirements of next-generation systems.

As mentioned above, the “delay spread” or difference in propagationdelays among the multiple components of received multipath signals hasalso tended to constitute a principal impediment to improved capacityand performance in wireless communication systems. It has been reportedthat when the delay spread exceeds approximately ten percent (10%) ofthe symbol duration, the resulting significant intersymbol interference(ISI) generally limits the maximum data rate. This type of difficultyhas tended to arise most frequently in narrowband systems: such as theGlobal System for Mobile Communication (GSM).

The existence of CCI also adversely affects the performance and capacityof cellular systems. Existing cellular systems operate by dividing theavailable frequency channels into channel sets, using one channel setper cell, with frequency reuse. Most time division multiple access(TDMA) systems use a frequency reuse factor of 7, while most codedivision multiple (CDMA) systems use a frequency reuse factor of 1. Thisfrequency reuse results in CCI, which increases as the number of channelsets decreases (i.e., as the capacity of each cell increases). In TDMAsystems, the CCI is predominantly from one or two other users, while inCDMA systems there may exist many strong interferers both within thecell and from adjacent cells. For a given level of CCI, capacity can beincreased by shrinking the cell size, but at the cost of additional basestations.

The impairments to the performance of cellular systems of the typedescribed above may be at least partially ameliorated by usingmulti-element antenna systems designed to introduce a diversity gaininto the signal reception process. There exist at least three primarymethods of effecting such a diversity gain through decorrelation of thesignals received at each antenna element: spatial diversity,polarization diversity and angle diversity. In order to realize spatialdiversity, the antenna elements are sufficiently separated to enable lowfading correlation. The required separation depends on the angularspread, which is the angle over which the signal arrives at the receiveantennas.

In the case of mobile subscriber units (e.g, handsets) surrounded byother scattering objects, an antenna spacing of only one quarterwavelength is often sufficient to achieve low fading correlation. Thispermits multiple spatial diversity antennas to be incorporated within ahandset, particularly at higher frequencies (owing to the reduction inantenna size as a function of increasing frequency). Furthermore, dualpolarization antennas can be placed close together, with low fadingcorrelation, as can antennas with different patterns (for angle ordirection diversity).

Although increasing the number of receive antennas enhances variousaspects of the performance of multi-antenna systems, the necessity ofproviding a separate RF chain for each transmit and receive antennaincreases costs. Each RF chain is generally comprised of a low noiseamplifier, filter, downconverter, and analog to digital to converter(A/D), with the latter three devices typically being responsible formost of the cost of the RF chain. In certain existing single-antennawireless receivers, the single required RF chain may account for inexcess of 30% of the receiver's total cost. It is thus apparent that asthe number of receive antennas increases, overall system cost and powerconsumption may dramatically increase. It would therefore be desirableto provide a technique that effectively provides additional receiveantennas without proportionately increasing system costs and powerconsumption.

SUMMARY OF THE INVENTION

In one embodiment, the invention can be characterized as a method, andmeans for accomplishing the method, for receiving a signal, the methodincluding receiving K replicas of the signal, each of the K replicasbeing received by one of a corresponding K antennas so as to therebygenerate K received signal replicas; processing each of the K receivedsignal replicas using one of N orthogonal sequences, thereby generatingK processed signal replicas, wherein N is less than K; orthogonallymultiplexing the K processed received signal replicas into a multiplexedsignal provided to a signal processing chain; downconverting, within thesignal processing chain, the multiplexed signal into a basebandmultiplexed signal; and transforming the baseband multiplexed signalinto K separate signals wherein each of the K separate signalscorresponds to one of the K replicas of the signal.

In another embodiment, the invention may be characterized as apparatusfor receiving a signal comprising: K antenna elements, wherein the Kantenna elements are arranged to receive one of a corresponding Kreplicas of the signal and thereby generate K received signal replicas;a signal processing chain; a first multiplexer configured to receive Nof the K received signal replicas and generate a first set of N channelsignals, wherein each of the N channel signals is spread according to acorresponding one of N orthogonal sequences and corresponds to one ofthe N received signal replicas; a second multiplexer configured toreceive M of the K received signal replicas and generate a second set ofM channel signals, wherein each of the M channel signals is spreadaccording to one of the N orthogonal sequences and corresponds to one ofthe M received signal replicas; a summing portion coupled between thesignal processing chain and the first and second multiplexers, whereinthe summing portion is configured to combine the first set of N channelsignals and the second set of M channel signals into a multiplexedsignal and provide the multiplexed signal to the signal processingchain; a downconversion module configured to downconvert, within thesignal processing chain, the multiplexed signal to a basebandmultiplexed signal; and a signal recovery module coupled to the signalprocessing chain, wherein the signal recovery module is configured toreceive the baseband multiplexed signal and provide K separate signalsfrom the baseband multiplexed signal, wherein each of the K separatesignals corresponds to one of the K replicas of the signal.

In a further embodiment, the invention may be characterized as a methodfor multiplexing K channels on to a receiver chain, the K channelsincluding N channels corresponding to N antenna elements and M channelscorresponding to M antenna elements, the method comprising: spreadingeach of the N channels according to a corresponding one of N orthogonalsequences so as to form N spread channels; overlaying a first scramblingsequence on to the N spread channels so as to form a first set of Nchannels; spreading each of the M channels according to one of the Northogonal sequences so as to form M spread channels; overlaying asecond scrambling sequence on to the M spread channels so as to form asecond set of M channels; combining the first set of N channels and thesecond set of M channels so as to form K multiplexed channels, andproviding the K multiplexed channels to the receiver chain.

In yet another embodiment the invention may be characterized as a methodfor separating K symbol streams, each of the K symbol streams beingconveyed by K respective orthogonally spread channels in a receiverchain, the K channels including a first set of N channels and a secondset of M channels, each of the N channels being spread according to acorresponding one of N orthogonal sequences and each of the M channelsbeing spread according to one of the N orthogonal sequences, the methodcomprising: despreading the first set of N channels so as to generate Nseparate channels; detecting, from the N separate channels, a set of Nsymbols wherein each of the N symbols is conveyed by a corresponding oneof the N channels; generating a first interference signal due to thefirst set of N channels based upon the set of N symbols; subtracting theinterference signal from the second set of M channels; despreading thesecond set of M channels so as to generate M separate channels;detecting, from the M separate channels, a set of M symbols wherein eachof the M symbols is conveyed by a corresponding one of the M channels;and providing K separate symbols wherein the K separate symbols includethe set of N symbols and the set of M symbols.

In yet a further embodiment, the invention may be characterized as amethod for receiving a signal with an antenna array comprising:receiving K replicas of the signal, each of the K replicas beingreceived by one of a corresponding K antenna elements of the antennaarray, wherein the K replicas include N replicas and Mother replicas ofthe received signal; multiplexing the N replicas and the M replicas ofthe signal into a multiplexed signal provided to a single processingchain; removing interference due to the N signals from the multiplexedsignal; demultiplexing, after the interference due to the N signals isremoved, the M signals from the multiplexed signal, thereby generating Mdetected signals; removing interference due to the M signals from themultiplexed signal; demultiplexing, after the interference due to the Msignals is removed, the N signals from the multiplexed signal, therebygenerating N detected signals.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a block diagram of a conventional diversity receiver in whichthe signals received by multiple antenna elements are weighted andcombined in order to generate an output signal;

FIG. 2 is a block diagram of a conventional spatial-temporal (ST)filtering arrangement;

FIG. 3 is a representation of a multiple-input/multiple-output antennaarrangement within a wireless communication system;

FIG. 4 is a block diagram of an antenna processing system configured toreduce the number of separate signal processing chains associated withan antenna array;

FIG. 5 is a high-level block diagram of a multi-antenna receiver systemimplemented in accordance with one embodiment of the present invention;

FIG. 6 is a flow chart illustrating steps carried out by the amulti-antenna receiver system of FIG. 5 to receive a signal withmultiple antennas according to one embodiment;

FIG. 7 is a block diagram of a multi-antenna receiver system configuredto implement iterative multi-stage detection in accordance with oneembodiment of the antenna system of FIG. 5;

FIG. 8 is a flowchart depicting steps carried out by the multi-antennareceiver system of FIG. 7 when carrying out the iterative multistagedetection process according to one embodiment of the present invention;

FIG. 9 is a graph depicting simulated results of the iterativemultistage detection process carried out by the multi-antenna receiversystem of FIG. 7 with a spreading factor of (N) selected to be 16 and anumber of channels selected to be N+1;

FIG. 10 is a graph depicting simulated results of the iterativemultistage detection process carried out by the multi-antenna receiversystem of FIG. 7 with a spreading factor of (N) selected to be 16 and anumber of channels selected to be N+2;

FIG. 11 is a graph depicting simulated results of the iterativemultistage detection process carried out by the multi-antenna receiversystem of FIG. 7 with a spreading factor of (N) selected to be 16 and anumber of channels selected to be N+3;

FIG. 12 is a graph depicting simulated results of the iterativemultistage detection process carried out by the multi-antenna receiversystem of FIG. 7 with a spreading factor of (N) selected to be 16 and anumber of channels selected to be N+7; and

FIG. 13 is a graph depicting simulated results of the iterativemultistage detection process carried out by the multi-antenna receiversystem of FIG. 7 with a spreading factor of (A) selected to be 7 and anumber of channels selected to be N+1.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, various aspects of the present inventionwill be described. However, it will be apparent to those skilled in theart that the present invention may be practiced with only some or allaspects of the present invention. For purposes of explanation, specificnumbers, materials and configurations are set forth in order to providea thorough understanding of the present invention. However, it will alsobe apparent to one skilled in the art that the present invention may bepracticed without the specific details. In other instances, well knownfeatures are omitted or simplified in order not to obscure the presentinvention.

Various operations will be described as multiple discrete stepsperformed in turn in a manner that is most helpful in understanding thepresent invention, however, the order of description should: not beconstrued as to imply that these operations are necessarily orderdependent, in particular, the order the steps are presented.Furthermore, the phrase “in one embodiment” will be used repeatedly,however the phrase does not necessarily refer to the same embodiment,although it may.

The present invention according to several embodiments allows K signalchannels associated with K respective antenna elements to beorthogonally multiplexed onto a signal processing chain of a receiverusing less than K orthogonal sequences. As a consequence, a receiverusing a single receive chain characterized by a spreading factor of N,which, would otherwise be limited to N antenna elements, may incorporatemore than N antenna elements; thus increasing the capacity of thereceiver.

The present invention is applicable to mobile devices and alsoinfrastructure elements (e.g., base stations and access points). Inaddition, the present invention is applicable to nearly all knownwireless standards and modulation schemes (e.g., GSM, CDMA2000, WCDMA,WLAN, fixed wireless standards, OFDM and CDMA). As will be describedbelow, various advantages offered by the present invention derive fromthe multiplexing of the signals received from a number of antennaelements onto a common receive chain processing path in order to reduceoverall power consumption and cost.

In order to facilitate appreciation of the principles of the invention,a brief overview of various conventional multi-element antenna systemsdesigned to mitigate delay spread, interference and fading effects isprovided with reference to FIGS. 1-4.

Referring first to FIG. 1, shown is a block diagram of a conventionaldiversity receiver 100 in which the signals received by multiple antennaelements are weighted and combined in order to generate an outputsignal. Shown in the conventional diversity receiver 100 are acollection of M antenna elements 102, and coupled with each respectiveantenna element are parallel receive chains 104, 106, 108 that includerespective weighting portions 110, 112, 114. The receive chains 104,106, 108 all couple with a combiner 116 disposed to produce a combinedsingle 118.

An array of M antenna elements generally provides an increased antennagain of “M.” Such an array also provides a diversity gain againstmultipath fading dependent upon the correlation of the fading among theantenna elements. In this context the antenna gain is defined as thereduction in required receive signal power for a given average outputsignal-to-noise ratio (SNR), while the diversity gain is defined as thereduction in the required average output SNR for a given bit error rate(BER) with fading.

For interference mitigation, each of the M antenna elements 102 areweighted at the respective weighting portions 110, 112, 114 and combinedin the combiner 116 to maximize signal-to-interference-plus-noise ratio(SINR). This: weighting process is usually implemented in a manner thatminimizes mean squared error, and utilizes the correlation of theinterference to reduce the interference power.

Turning now to FIG. 2, a block diagram is shown of a conventionalspatial-temporal (ST) filtering arrangement 200. Shown are a firstantenna 202 and a second antenna 204 respectively coupled to a firstlinear equalizer 206 and a second linear equalizer 208. Outputs of eachof the first and second linear equalizers 206, 208 are coupled to acombiner 210, and an output of the combiner 201 is coupled to anMLSE/DFE portion 212.

The filtering arrangement of FIG. 2 is designed to eliminate delayspread using joint space-time processing. In general, since the CCI isunknown at the receiver, optimum space-time (ST) equalizers, either inthe sense of a minimum mean square error (MMSE) or maximumsignal-to-interference-plus-noise ratio (SINR), typically include awhitening filter. For example, linear equalizers (LE) 206, 208 thatwhiten the CCI both spatially and temporally, and the filteringarrangement of FIG. 2 are typical of such systems. As shown in FIG. 2,the linear equalizers (LE) 206, 208 are followed by a non-linear filterthat is represented by the MLSE/DFE portion 212, which is implementedusing either a decision feedback equalizer (DFE) or maximum-likelihoodsequence estimator (MLSE).

As is known to one of ordinary skill in the art, the turbo principle canalso be used to replace the non-linear filters with superiorperformance, but higher computational complexity. Using ST processing(STP) techniques, SNR gains of up to 7 dB and SINR gains of up to 21 dBhave been reported with a modest number of antenna elements.

Referring next to FIG. 3, shown is a generic representation of amultiple-input/multiple-output antenna arrangement within a wirelesscommunication system 300. Shown are a transmitter (TX) 302 coupled tomultiple transmit antennas 304, which are shown transmitting a signalvia time varying obstructions 306 to multiple receive antennas 308coupled to a receiver (RX) 310.

In addition to multiple-input/multiple-output antenna (MIMO)arrangements, other antenna arrangements may be categorized, based uponthe number of “inputs” and “outputs” to the channel linking atransmitter and receiver, as follows:

-   -   Single-input/single-output (SISO) systems, which include        transceivers (e.g., mobile units and a base station) with a        single antenna for uplink and down link communications.    -   Multi-input/single-output (MISO) systems, which include one or        more receivers, which downlink via multiple antenna inputs, and        one or more transmitters, which uplink via a single antenna        output.    -   Single-input/multi-output (SIMO) systems, which include one or        more receivers, which downlink via a single antenna input, and        one or more transmitters, which uplink via multiple antenna        outputs.

One aspect of the attractiveness of multi-element antenna arrangements,particularly MIMOs, resides in the significant system capacityenhancements that can be achieved using these configurations. Assumingperfect estimates of the applicable channel at both the transmitter andreceiver are available, in a MIMO system with M receive antennas thereceived signal decomposes to M independent channels. This results in anM-fold capacity increase relative to SISO systems. For a fixed overalltransmitted power, the capacity offered by MIMOs scale with increasingSNR for a large, but practical, number of M of antenna elements.

In the particular case of fading multipath channels, it has been foundthat the use of MIMO arrangements permits capacity to be scaled bynearly M additional bits/cycle for each 3-dB increase in SNR. This MIMOscaling attribute is in contrast to a baseline configuration,characterized by M=1, which by Shannon's classical formula scales as onemore bit/cycle for every 3-dB of SNR increase. It is noted that thisincrease in capacity that MIMO systems afford is achieved without anyadditional bandwidth relative to the single element baselineconfiguration.

However, widespread deployment of multi-element antenna arrangements inwireless communication systems (particularly within wireless handsets)has been hindered by the resultant increase in complexity and associatedincreased power consumption, cost and size. These parameter increasesresult, at least in part, from a requirement in many proposedarchitectures that a separate receiver chain be provided for eachantenna element.

One technique which has been developed to utilize multiple antennaelements with a reduced number of signal processing chains includesmultiplexing signals from multiple antennas on to a single processingchain as disclosed in a related copending U.S. application Ser. No.10/606,371, entitled REDUCED-COMPLEXITY ANTENNA SYSTEM USING MULTIPLEXEDRECEIVE CHAIN PROCESSING, filed Jun. 27, 2003, which is assigned to theassignee of the present application and is incorporated herein byreference in its entirety.

Referring next to FIG. 4, shown is an antenna processing system 400configured to reduce the number of separate signal processing chainsassociated with an antenna array in accordance with the above-identifiedU.S. application Ser. No. 10/606,371. As shown, the antenna processingsystem 400 includes N antennas 402, 404, 406, 408 coupled to amultiplexer 410, which is coupled to a single signal processing chain416. The multiplexer 40 is configured to orthogonally multiplex Nchannels (corresponding to the N antennas 402 onto the signal processingchain 416, and is characterized by a spreading factor of N: that is, themultiplexer 410 utilizes N orthogonal sequences of length N.

In operation, each of the N antennas 402 receives an incident RF signalat spatially distinct locations and provides a replica of the incidentRF signal to the multiplexer 410. As a consequence, the multiplexer 410receives N replicas of the incident RF signal. The multiplexer 410 thenorthogonally multiplexes the N replicas of the incident RF signal on tothe single processing chain 416 to form a multiplexed signal comprisingN multiplexed channels. Because each of the N channels is assigned adifferent orthogonal code during multiplexing, a manageable level ofinterference exists between the N multiplexed channels within the signalprocessing chain 416.

Once provided to the signal processing chain 416, the multiplexed signalis then frequency downconverted, filtered and converted from analog forminto a digital multiplexed signal. The digital multiplexed signal isthen demultiplexed by a demultiplexor 436 into N separate signals thatcorrespond to the N replicas of the signal received at the N antennas.The N separate signals are then subjected to conventional spatialprocessing.

Although the antenna processing system 400 provides substantial cost andpower savings over systems employing a separate signal processing chainfor each antenna, in some applications it would be desirable if theantenna processing system 400 could support more than N channels, i.e.,more than N antennas. Because each of the N orthogonal sequences isalready used by one of the N antennas, however, an additional channelmultiplexed onto the signal processing chain 416 would not be orthogonalto at least one of the N multiplexed channels. As a consequence, theadditional channel would both impart deleterious interference on one ormore of the N multiplexed channels and receive substantial interferencefrom at least one of the N multiplexed channels.

Overview

As is described in further detail below, the iterative multistagedetection technique of the present invention may be utilized to providea cost effective means to increase the capacity of wireless systemsdeploying multi-element antenna arrangements. In one aspect of theinvention, an antenna system is configured to orthogonally multiplex Kchannels onto a single signal processing chain using N orthogonalsequences of length N. The K channels include a first set of N channelsand a second set of M channels (the M channels being separate anddistinct from the N channels), where K=N+M and in an exemplaryembodiment M<N. Therefore, a multiplexed signal is created on the signalprocessing chain, which includes a first set of N multiplexed channelsand a second set of M multiplexed channels.

In accordance with one aspect of the invention, an iterative process isused to receive the multiplexed signal. In a first iteration,interference from the first set of N channels imparted on the second setof M channels is removed from the multiplexed signal, thereby enablingthe symbol values associated with the second set of M channels to bereliably estimated. In a second iteration, interference from the secondset of M channels imparted on the first set of N channels is removedfrom the first set of N channels, thereby enabling the symbol valuesassociated with the first set of N channels to be reliably estimated. Inthis way, K channels may be multiplexed on to a single receiver chainwith less than K orthogonal sequences, and then reliably estimated afterprocessing (e.g., after down conversion and digitization) by thereceiver chain.

Referring next to FIG. 5, shown is a high-level block diagram of areceiver 500 incorporating an antenna system in accordance with anexemplary embodiment of the present invention. While referring to FIG. 5simultaneous reference will be made to FIG. 6, which is a flow chartillustrating steps carried out by the antenna system 500 to receive asignal with multiple antennas according to the present embodiment. Asshown, the antenna system 500 includes an N channel multiplexer 502, andan M channel multiplexer 507. The N channel multiplexer 502 isconfigured to receive N replicas of a signal with a set of N respectiveantennas 505, and the M channel multiplexer 507 is configured to receiveM replicas of the signal with a set of M respective antennas 508.Collectively the N and M channel multiplexers 502, 507 receive K signalreplicas (i.e., K=M+N) (Step 600).

In operation, the N channel multiplexer 502 and the M channelmultiplexer 507 collectively multiplex, in cooperation with thesummation module 530, the K received signal replicas on to the signalprocessing chain 510 (Step 602). In an exemplary embodiment, the Nchannel multiplexer 502 assigns each of the N replicas of the signal acorresponding one of N orthogonal time sequences to form a firstcomposite signal. The N channel multiplexer 502 then overlays a commonfirst PN scrambling sequence on to the first composite signal so as toform a first set of N scrambled signals 512 (also referred to herein asa “first set of N channels” or “set #1 channels”).

Similarly, the M channel multiplexer 507 assigns each of M of the Northogonal sequences to a corresponding one of the M replicas of thesignal to form a second composite signal. In other words, the M channelmultiplexer 507 reuses a subset of the N orthogonal sequences to formthe second composite signal. The second multiplexer 507 then overlays asecond PN scrambling sequence on to the second composite signal so as toform a second set of M scrambled signals 514 (also referred to herein asa “second set of M channels” or “set #2 channels”). The summation module530 then combines the first set of N channels 512 and second set of Mchannels 514 so as to form a multiplexed signal 516, which is providedto the signal processing chain 510. Within the signal processing chain510 the multiplexed signal 516 is downconverted by a downconversionmodule 540 (e.g., a mixer to convert from RF to baseband frequency),filtered by a filter 542 and digitized by an analog to digital converter544.

Assuming time synchronization is established throughout the antennasystem 500, there exists substantially no mutual interference in theprocessing chain 510 among the first set of N channels. That is, thefirst set of N channels only experience interference as a consequence ofthe second set of M channels. The interference power (i.e., in-phase andquadrature phase energy) associated with each channel of the second setof M channels (assuming that useful signal power is normalized by 1) is1/N. It follows that the total interference power experienced by thefirst set of N channels is MIN. As long as M remains relatively smallcompared to N it is possible to make at least preliminary decisions asto the values of the symbols transmitted via the first set of Nchannels. However, since each channel of the second set of M channelsexperiences an interference power of N (1/N) or 1 as a consequence ofthe first set of N channels, the symbol values associated with thesecond set of M channels may not be directly estimated with anyreasonable degree of certainty through straightforward application ofconventional techniques.

As shown in FIG. 5, after the multiplexed signal 516 is downconverted,filtered and digitized, the resultant baseband multiplexed signal 546 isprovided to a signal recovery module 550. In general, the signalrecovery module 550 receives the baseband multiplexed signal 546 andrecovers K separate signals, which correspond to the K received signalreplicas received by the K antennas.

Initially, the signal recovery module 550 receives the basebandmultiplexed signal 546, and removes interference imparted by the firstset of N channels on the second set of M channels from the multiplexedsignal so as to generate a preliminary estimate of the symbol streamscarried by the second set of M channels (Step 604). In an exemplaryembodiment, the signal recovery module 550 determines the interferenceimparted by the first set of N channels upon the second set of Mchannels by demultiplexing the first set of N channels from the basebandmultiplexed signal 546, establishing preliminary values of the symbolsreceived through the first set of N channels and then synthesizing anaggregate interference signal associated with the first set of Nchannels based upon these preliminary symbol values. The aggregateinterference signal also provides an estimate of the symbol streamsconveyed via the first set of N channels.

After interference from the first set of N channels is removed from thebaseband multiplexed signal 546, the signal recovery module 550demultiplexes M separate signals (corresponding to the M replicas of thesignal) from the preliminary estimate of the second set of M channels(Step 606). Because interference from the first set of N channels isfirst removed from the baseband multiplexed signal 546 to form thepreliminary estimate of the second set of M channels, the signalrecovery module 550 may reliably estimate the symbol values associatedwith the M separate signals.

During a second signal recovery iteration, interference from the secondset of M channels is then removed from the estimates of the symbolstreams corresponding to the first set of N channels (produced duringStep 604) in order to provide a revised estimate of these symbol streams(Step 608). Since the preliminary symbol values of the first set of Nchannels are initially made in the presence of the interference from thesecond set of M channels, this step removes the interference originatingfrom the second set of M channels so the symbol values of the first setof N channels may be more reliably estimated.

The signal recovery module 550 then demultiplexes the revised estimateof the first set of N channels into N separate signals (corresponding tothe N replicas of the incident RF signal) from the baseband multiplexedsignal 546 (Step 610).

The signal recovery module 550 then provides K separate signals (i.e.,the N separate signals and the M separate signals) to a signalprocessing portion 570 for further processing. The signal processingportion 570 may include additional spatial and iterative (turbo)processing, as well as de-interleaving (bit and/or symbol level) andchannel decoding.

Turning now to FIG. 7, a block diagram is provided of a multi-antennareceiver system 700 configured to implement iterative multi-stagedetection in accordance with the present invention. The receiver system700 includes a multistage receiver unit 710 disposed to receive andprocess RF signal energy collected by a K element antenna, array 712. Asshown, the receiver system also includes a signal recovery module 714,which functions to separate K multiplexed channels. In this way, Ksymbol streams received at the K element antenna array 712, and conveyedby the K multiplexed channels, may be separated and recovered at thesignal recovery module 714. As shown, the signal recovery portion 714includes an N channel recovery portion 716 and an M channel recoveryportion 718, which cooperate to carry out the functions of the signalrecovery module 714. Specifically, the N channel recovery portion 716 incooperation with the M channel recovery portion 718 function to provideN separate symbol streams and M separate symbol streams, respectively.Together the N separate symbol streams and the M separate symbol streamsprovide K separate symbol streams that correspond to (e.g., closelyestimate) the K symbol streams received at the K element antenna array712.

As shown, the antenna array 712 includes a first set of Nspatially-separated receiving antennas 704 and a second set of Mspatially-separated receiving antennas 708. The N antennas 704 and the Mantennas 708 couple an RF signal comprised of a first set of N channelsand a second set of M channels into the receiver unit 710. The receivedRF signal is passed through the N antennas 704 to a set #1 channelspreading module 720 and is passed through the M antennas 708 to achannel set #2 spreading module 727. Within the spreading module 720,the N received signal replicas a₁, a₂, . . . a_(N) received from the Nantenna elements 704 ₁, 704 ₂, and 704 _(N) are each spread by adifferent one of N orthogonal sequences of length N associated with thefirst set of N channels.

Similarly, within the spreading module 727, the M received signalreplicas a_(N+1), a_(N+2), . . . a_(N+M) received from the M antennaelements 70 _(N+1), 708 _(N+2), . . . 708 _(N+M) are each spread by adifferent one of M orthogonal sequences of length N associated with thesecond set of M channels. A set of N spread signals 730 are provided bythe spreading module 720 to a summation module 731 operative to providea composite set #1 channel signal to a first mixer element 732. In likemanner a set of M spread signals 737 are provided by the spreadingmodule 727 to a summation module 736 operative to provide a compositeset #2 channel signal to a second mixer element 740.

As shown in FIG. 7, the composite set #1 channel signal is scrambled atthe first mixer element 732 using a first PN scrambling sequence P₁ andthe composite channel set #2 signal is scrambled at the second mixerelement 740 using a second PN scrambling sequence P₂. The resultant set#1 channel and set #2 channel scrambled signals (also referred to hereinas a first set of N channel signals and a second set of M channelsignals, respectively) are combined within a summation module 744 inorder to form a multiplexed signal 745 which includes the first set of Nchannel signals and a second set of M channel signals. Within an RFprocessing module 778 the multiplexed signal 745 is filtered,down-converted from RF, and digitized to reform the multiplexed signal745 as a baseband multiplexed signal 746 composed from received samplesat baseband frequencies.

The baseband multiplexed signal 746 output of the RF processing module778 is provided to a buffer 749 in the signal recover module 714, andthe buffer 749 is switchably coupled to a baseband mixer element 752 viaa switch 750.

As shown, the complex conjugate P₁* of the first PN scrambling sequenceP₁ is also applied to the baseband mixer element 752 which, incooperation with a set #1 channel despreading module 756, serves todespread the received first set of N channel signals. In particular,within the despreading, module 756 the complex conjugates of each of theN orthogonal time sequences are each used to complete the despreading ofthe N baseband signal streams 760 received from the baseband mixerelement 752. That is, each of the N baseband signals is despread by oneof the N orthogonal time sequences. In an exemplary embodiment, thedespreading module 756 includes a bank of N complex correlators whichare matched to the N channels in the first set of N channels. The set ofN despread baseband signals from the despreading module 756 are thenpassed through a corresponding set of N threshold detectors 767, whichyields an initial estimate of the current symbol values for each of thereceived first set of N channel signals (i.e., â₁, â₂, . . . , â_(N)).

In accordance with the invention, the estimated symbol values â₁, â₂, .. . , â_(N) for the first set of N channel signals are used tosynthesize an interference signal intended to replicate the basebandsignal waveform of the received first set of N channel signals.Specifically, the estimated symbol values â₁, â₂, . . . , â_(N) of firstset of N channel signals are processed by a re-spreading module 768operative to spread each such value using the applicable one of the Northogonal time sequences. The resultant re-spread set of N channelsignals are then combined within a summation module 772 in order toproduce a composite re-spread signal. As shown, the composite re-spreadsignal is scrambled within mixer element 776 using the first PN sequenceP₁, thereby yielding a regenerated set of N channel signals 777, whichis provided as an interference signal 780 to a difference element 782 inthe M channel recovery portion 718. The regenerated set of N channelsignals 777 is also provided to an adder element 798 for use during asecond iteration.

The difference element 782 is arranged to receive the interferencesignal 780 for the first set of N channels and the baseband multiplexedsignal 746 from the delay element 787. The output of difference element782, which approximates the baseband signal waveform of the second setof M channel signals, is descrambled by mixer element 786 using thecomplex conjugate P₂* of the second PN sequence P₂. The resultantdescrambled signal is then despread within the despreading module 788 byeach of the M orthogonal time sequences associated with the second setof M channels. In an exemplary embodiment, the despreading module 788includes a bank of M complex correlators which are matched to the Mchannels in the first set of M channels. The resulting set of M despreadbaseband signals from the despreading module 788 are applied to a set ofM threshold detectors 790, which yield estimates of current symbolvalues â_(N+1), â_(N+2), . . . , â_(N+M) for each of the second set of Mchannel signals. The estimated symbol values â_(N+1), â_(N+2), . . . ,â_(N+M) of the second set of M channel signals are processed by a secondrespreading module 792 operative to spread each such value using theapplicable one of the M orthogonal time sequences (i.e., the subset ofthe N orthogonal sequences used by the channel set #2 spreading module727). The resultant re-spread set of M channel signals are then combinedwithin a summation module 794 in order to produce a second compositere-spread signal. As shown, the second composite re-spread signal isscrambled within mixer element 796 using the second PN sequence P₂,thereby yielding a regenerated set of M channel signals 797, which isprovided to an adder element 798.

As a consequence, K separate estimated symbol values, i.e., theestimated symbol values â₁, â₂, . . . , â_(N) of first set of N channelsignals and the estimated symbol values â_(N+1), â_(N+2), . . . ,â_(N+M) of the second set of M channel signals, are provided during afirst iteration.

The adder element 798 combines the regenerated set of N channel signals777 and the regenerated set of M channel signals 797 to form aregenerated baseband multiplexed signal 799, which according to anexemplary embodiment, is processed during a second iteration asdiscussed herein to produce a more accurate set of K separate symbolvalues.

The iterative interference removal process will be better understoodwith a brief consideration of the effect of spreading and scrambling theN and M signal replicas received at the N antennas 505 and the Mantennas 508, respectively. To begin, suppose {W_(i)|i=1,2, . . . ,N}designate the N binary orthogonal time sequences used in spreading thefirst set of N channel signals. The i^(th) of the sequences may beexpressed as W_(i)=(w_(i,1), w_(i,2), . . . , w_(i,N)), where w₁,designates the m^(th) chip of the sequence W₁. Note that each of thesequences W_(i) is independent of the symbol index, since each sequencerepeats from one symbol to the next. Next, suppose that {P_(n)|i=1,2}designate the first and second PN scrambling sequences that overlay thetime orthogonal sequences of the first set of N channel signals and thesecond set of M channel signals. Although the first and second PNsequences P₁ and P₂ do not repeat, the symbol index may also be removedfrom the PN sequences since the signal processing of concern ismemoryless. That is, detection of a current symbol does not involvesignal samples from previous and future symbols. Consequently, each ofthe PN sequences may be expressed as P_(n)=(p_(n,1), p_(n,2), . . . ,p_(n,N)). The resulting composite sequences for channel i (i=1,2, . . .,N) and channel N+k (k=0,2, . . . ,M) are denoted (α_(i,1), α_(i,2), . .. , α_(i,N)) and (β_(i,1), β_(i,2), . . . , β_(i,N)), respectively, withα_(i,m)=w_(i,m)p_(i,m) and β_(k,m)=w_(k,m)p_(2,m) for m=1,2, . . . ,N.

Since it will be desired to divide the power of the synthesizedinterference signal evenly over the in-phase and quadrature componentsof the useful signal (irrespective of carrier phases), complex-valued PNsequences are considered; that is, the chips p_(n,m) randomly assumevalues from the set {exp(jπ/2), exp(−jπ/2), exp(j3π/2), exp(−j3π/2)}.

Referring next to FIG. 8, shown is a flowchart depicting steps carriedout by the multi-antenna receiver system 700 when carrying out theiterative multistage detection process according to one embodiment ofthe present invention.

As mentioned earlier, the interference affecting first set of N channelsis limited. Accordingly, initial estimates of the symbol values of thefirst set of N channel signals may be made using a threshold detectorimmediately following despreading by the corresponding composite chipsequences (Step 802). This step of the detection process yields thefollowing set of set of initial decisions for the first set of N channelsignals: â₁, â₂, . . . , â_(N).

The initial decisions â₁, â₂, . . . , â_(N) are then used to synthesizean estimated interference caused by the first set of N channels withrespect to the second set of M channels (Step 804). This estimatedinterference is then subtracted from the baseband signal energy of themultiplexed signal (Step 806), thereby yielding a difference signalcorresponding to an estimate of the second set of M channel signals (atbaseband). Assuming each of the second set of M channels may beidentified by an index N+k (k=1,2, . . . , M), the total interferencefrom the first set of N channels may be expressed as: $\begin{matrix}{I_{N + k} = {\frac{1}{N}{\sum\limits_{i = 1}^{N}{a_{i}\left\lbrack {\sum\limits_{j = 1}^{N}\left( {\alpha_{i,j}\beta_{k,j}^{*}} \right)} \right\rbrack}}}} & (1)\end{matrix}$where α_(i) is the data symbol of the i^(th) channel during the currentsymbol interval. Each term in the outer sum in (1) represents theinterference from one of the N channels. Since the chip sequences(α_(i,1), α_(i,2), . . . , α_(i,N)) and (β_(i,1), β_(i,2), . . . ,β_(i,N)) are known to the receiver, I_(N+k) can be estimated once thesymbol decisions corresponding to channels 1 to N of the first set of Nchannels have been made. This estimate I_(N+k) is subtracted from thecorresponding signal at a correlator output before sending the result toa threshold detector.

After the estimated interference caused by the first set of N channelsis removed from the multiplexed signals, the symbol values of thereceived second set of M channel signals are estimated using thethreshold detector 790 immediately following despreading by thedespreading module 788 (Step 808). This step of the detection processyields the following set of symbol decisions for the second set of Nchannel signals: â_(N+1), â_(N+2), . . . , â_(N+M).

If all initial decisions â₁, â₂, . . . , â_(N) for the first set of Nchannel signals are made correctly at Step 802, complete interferencecancellation effectively occurs at Step 806 and substantially no mutualinterference between the first set of N channels and the second set of Mchannels will remain when the symbol values of the second set of Mchannel signals are estimated at Step 808. Each incorrect decision withregard to â₁, â₂, . . . , â_(N) yielded in Step 802 will, however, causethe corresponding term in I_(N+k) to increase and thereby reduce thelikelihood of accurate estimation of the second set of M channels.

In an exemplary embodiment, to improve the accuracy of detection of thesymbols conveyed by the second set of M channels, a second iteration ofmay be performed. Specifically, the symbol decisions â_(N+1), â_(N+2), .. . , â_(N+M) made for the second set of M channels in the firstiteration are used to synthesize interference of the second set of Mchannel signals (Step 810). The interference of the second set of Mchannels is then subtracted from the first set of N channel signals(Step 812).

During the first iteration, the baseband multiplexed signal 746 producedby the RF processing module 778 is buffered within buffer 749 anddirectly coupled therefrom to the mixer element 752 via switch 750.During the second and any subsequent iterations, the switch 750 is setto couple the regenerated baseband multiplexed signal 799 from theoutput of adder element 798 (obtained from mixer elements 776 and 796)to the baseband mixer element 752, while the while the buffer 749 isfilled with the incoming signal received from RF processing module 778.Ideally, all iterations are performed while the buffer 749 is updatedand completed before the buffer contents has been filled with a new RFsignal. In other words, the iterative processing is done within one bitinterval (i.e., within one bit duration), so that the size of the buffer749 remains manageable. In an exemplary embodiment, the iterativeprocessing is performed in a much shorter period than a bit duration,and when the buffer 749 is filled with a new set of bit samples, theprocessing of the new set of bit samples by the signal recovery module714 begins.

The interference from the second set of M channels in the k^(th) channelsignal (k=1,2, . . . ,N) is given by: $\begin{matrix}{I_{k} = {\frac{1}{N}{\sum\limits_{i = 1}^{N}{a_{N + i}\left\lbrack {\sum\limits_{j = 1}^{N}\left( {\beta_{i,j}\alpha_{k,j}^{*}} \right)} \right\rbrack}}}} & (2)\end{matrix}$This interference is synthesized by substituting â_(N+i) for a_(N+i) inEquation (2) above for i=1,2, . . . ,M . Since â_(N+i)=a_(N+i) with aprobability close to 1, the synthesized replica will generally bevirtually identical to the actual interference.

In an exemplary embodiment, during a second iteration, the regeneratedbaseband multiplexed signal 799 is descrambled and despread by thebaseband mixer element 752 and the set #1 channel despreading module 756to provide N despread baseband signals. The synthesized interference dueto the second set of M channel signals (determined during the firstiteration) from Step 810 is then subtracted from the k^(th) signal ofthe set of N despread baseband signals at the output of the despreadingmodule 756; thus effectively subtracting the interference of the secondset of M channels from the first set of N channel signals (Step 812).

The N interferenced-reduced signals produced by subtracting thesynthesized interference from the k^(th) signal of the set of N despreadbaseband signals is passed to the applicable threshold detector 767.This process is repeated for all of the first set of N channels todetermine a revised set of N symbol values of the first set of N channelsignals (Step 814).

A revised estimate of the interference caused by the first set of Nchannels with respect to the second set of M channels is then determinedbased upon the a revised set of symbol values (Step 816). In anexemplary embodiment, the revised symbol values of the first set of Nchannel signals are respread by the re-spreading module 768, recombinedwithin the summation module 772 and scrambled within mixer element 776using the first PN sequence P₁, thereby producing another interferencesignal 780, which is subtracted from the regenerated basebandmultiplexed signal at the difference element 782 so as to generate adifference signal corresponding to an estimate of the second set of Mchannel signals (Step 816).

Symbol value decisions for the second set of M channels are then madeduring the second iteration following subtraction of the interference ofthe first set of N channels (Step 818). In this regard the totalinterference experienced by the k^(th) channel of the second set of Mchannels (i.e., channel N+k) is given by Equation (1). After subtractingthe best available estimate of this total interference, the output ofthe despreading module 788 for the k^(th) channel of the second set of Mchannels is sent to the corresponding set of M threshold detectors 790,which produces a revised set of M symbol values of the first set of Mchannel signals.

Thus, after the second iteration, a revised set of N symbol values ofthe first set of N channel signals and a revised set of M symbol valuesof the second set of M channel signals is provided by the signalrecovery portion 714. Together such revised symbol values provide Kseparate symbol values, which correspond to K symbol streams in them Kreceived signal replicas received at the K element antenna array 712.

It has been found when the number of excess channels M is limited toapproximately 25% of the spreading factor N, execution of two or threeiterations yields sufficiently good performance that additionaliterations are unnecessary. As the number of excess channels Mapproaches 25% of N, performance has been found to be improved throughexecution of additional iterations.

Simulation Results

FIGS. 9-13 depict the results of various simulations of theabove-described iterative multi-stage detection process using two setsof orthogonal spreading sequences. In FIGS. 9-12 a spreading factor N of16 was employed, while in FIG. 13 a spreading factor N of 7 wasutilized. The number of “excess” channels M was selected to be 1, 2, 3and 7 in, FIGS. 9-12, respectively, and M was chosen to be 1 in the caseof FIG. 13. In addition, the simulations were executed exclusively atbaseband (no modulation or spectrum-shaping filtering were simulated),and an AWGN channel and synchronous operation (i.e., synchronous timespread) were assumed.

Referring to FIGS. 9-13, trace A represents the theoretical single userbound while trace B represents the performance of a single, uncodedchannel. (i.e., as “single user bound”) obtained through simulation. Inaddition, trace C represents the BER of the first set of N channelsprior to the performance of interference cancellation, trace Drepresents the BER of the second set of M channels prior to interferencecancellation, and trace E illustrates the overall BER (i.e., both thefirst set of N and the second set of M channels) prior to interferencecancellation. The BER of the first set of N channels following the firstiteration of interference cancellation is represented by trace F, theBER of the second set of M channels following the first iteration ofinterference cancellation is represented by trace G, and the overall BERfollowing the first iteration of interference cancellation isillustrated by trace H. Finally, the BER of the first set of N channelsfollowing the second iteration of interference cancellation isrepresented by trace I, the BER of the second set of M channelsfollowing the second iteration of interference cancellation isrepresented by trace J, and the overall BER following the seconditeration of interference cancellation is illustrated by trace K.

Although the simulations represented by FIGS. 9-13 demonstrate theeffectiveness of certain embodiments the inventive iterative multi-stagedetection technique, a value N of 16 (which results in deployment of atleast 16 antennas) may be impractical in certain applications. However,FIG. 13 demonstrates the effectiveness of the inventive technique undercurrently practical conditions (i.e., N=7, M=1).

In the simulations of FIGS. 9-13, complex PN scrambling sequences wereutilized. Specifically, the proposed π/2-separated complex scramblingsequence symbols were replaced with symbols separated by π/7 intervals.Other simulations based upon real-valued PN scrambling sequences havenot been found to yield performance of similar BER.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the invention.However, it will be apparent to one skilled in the art that the specificdetails are not required in order to practice the invention. In otherinstances, well-known circuits and devices are shown in block diagramform in order to avoid unnecessary distraction from the underlyinginvention. Thus, the foregoing descriptions of specific embodiments ofthe present invention are presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, obviously many modificationsand variations are possible in view of the above teachings. Theembodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with various modifications as are suited to theparticular use contemplated.

For example, an exemplary embodiment was described wherein a symbollevel procedure was performed to remove the interference due to thesecond set of M channel signals from the first set of N channel signalsat the correlator outputs of despreading module 756. It should berecognized that in an alternative embodiment, interference due to thesecond set of M channel signals from the first set of N channel signalsmay be removed at the chip level using a difference element as was doneto remove the interference due to the first set of N channels from thesecond set of M channel signals using the difference element 782.

Moreover, in the described exemplary embodiment, interference due to thefirst set of N channel signals was removed from the second set of Mchannel signals at the chip level using the difference element 782. Inan alternative embodiment, interference due to the first set of Nchannel signals may be removed from the second set of M channel signalson the symbol level at a correlator output after the despreading module788. In other words, interference from either the first set of Nchannels or the second set of M channels may be removed at either thechip or the symbol level and still be well within the scope of thepresent invention.

1. A system for receiving a signal, comprising: at least one processor that receive K replicas of the signal, each of the K replicas being received by one of a corresponding K antennas so as to thereby generate K received signal replicas; said at least one processor processes each of the K received signal replicas using one of N orthogonal sequences, thereby generating K processed signal replicas, wherein N is less than K; said at least one processor orthogonally multiplexes the K processed received signal replicas into a multiplexed signal provided to a signal processing chain; said at least one processor downconverts, within the signal processing chain, the multiplexed signal into a baseband multiplexed signal; and said at least one processor transforms the baseband multiplexed signal into K separate signals, wherein each of the K separate signals corresponds to one of the K replicas of the signal.
 2. The system of claim 1, wherein said at least one processor: assigns each of N of the K received signal replicas a corresponding one of the N orthogonal sequences so as to thereby generate a first composite signal; scrambles the first composite signal according to a first scrambling sequence so as to thereby generate a first set of N channel signals; assigns each of M of the K received signal replicas a corresponding one of M orthogonal sequences so as to thereby generate a second composite signal, wherein the M orthogonal sequences are a subset of the N orthogonal sequences; scrambles the second composite signal according to a second scrambling sequence so as to thereby generate a second set of M channel signals; and combines the first set of N channel signals and the second set of M channel signals so as to generate the multiplexed signal.
 3. The system of claim 2, wherein said at least one processor enables removing of interference due to the first set of N channel signals from the second set of M channel signals, and thereby generates M interference-reduced signals comprising a subset of the K separate signals.
 4. The system of claim 3, wherein said at least one processor enables removal of interference due to the second set of M channel signals from the first set of N channel signals, and thereby generates N interference-reduced signals comprising a subset of the K separate signals.
 5. The system of claim 3, wherein said at least one processor: despreads the first set of N channel signals so as to generate a set of N despread baseband signals; synthesizes an interference signal as a function of the set of N despread baseband signals; and subtracts the interference signal from the N despread baseband signals, and thereby removes interference due to the first set of N channel signals from the second set of M channel signals.
 6. The system of claim 5, wherein said at least one processor: passes each of the N despread baseband signals through a corresponding one of N threshold detectors so as to generate an estimated set of N symbol values for the first set of N channel signals; spreads each of the N symbol values according to a corresponding one of the N orthogonal sequences so as to generate a first baseband composite signal; and scrambles the first baseband composite signal according to the first scrambling sequence so as to synthesize the interference signal.
 7. The system of claim 4, wherein said at least one processor: despreads the first set of N channel signals so as to generate a set of N despread baseband signals; despreads the second set of M channels signals so as to generate a set of M despread baseband signals; and subtracts, from each of the N despread baseband signals, an interference signal synthesized as a function of the M despread baseband signals thereby removing interference due to the second set of M channel signals from the first set of N channel signals.
 8. The system of claim 7, wherein said at least one processor synthesizes the interference signal as a function of estimated symbol values generated from the M despread baseband signals.
 9. The system of claim 1, wherein the signal complies with a communication protocol selected from the group consisting of: orthogonal frequency division multiplexing (OFDM), time division multiple access (TDMA), code division multiple access (CDMA), gaussian minimum shift keying (GMSK), complementary code keying (CCK), quadrature phase shift keying (QPSK), frequency shift keying (FSK), phase shift keying (PSK), and quadrature amplitude modulation (QAM).
 10. A system for receiving a signal using K antennas, comprising: said one or more circuits receives K replicas of the signal, each of the K replicas being received by one of a corresponding one of the K antennas so as to thereby generate K received signal replicas; said one or more circuits processes of each of the K received signal replicas using one of N orthogonal sequences, and thereby generates K processed signal replicas, wherein N is less than K; said one or more circuits orthogonally multiplexes the K processed received signal replicas into a multiplexed signal provided to a signal processing chain; said one or more circuits downconverts, within the signal processing chain, the multiplexed signal into a baseband multiplexed signal; and said one or more circuits transforms the baseband multiplexed signal into K separate signals, wherein each of the K separate signals corresponds to one of the K replicas of the signal.
 11. The system of claim 10, wherein said one or more circuits: assigns each of N of the K received signal replicas a corresponding one of the N orthogonal sequences so as to thereby generate a first composite signal; scrambles the first composite signal according to a first scrambling sequence so as to thereby generate a first set of N channel signals; assigns each of M of the K received signal replicas a corresponding one of M orthogonal sequences so as to thereby generate a second composite signal, wherein the M orthogonal sequences are a subset of the N orthogonal sequences; scrambles the second composite signal according to a second scrambling sequence so as to thereby generate a second set of M channel signals; and combines the first set of N channel signals and the second set of M channel signals so as to generate the multiplexed signal.
 12. The system of claim 11, wherein said one or more circuits enables removing of interference due to the first set of N channel signals from the second set of M channel signals, and thereby generates M interference-reduced signals comprising a subset of the K separate signals.
 13. The system of claim 12, wherein said one or more circuits enables removal of interference due to the second set of M channel signals from the first set of N channel signals, and thereby generates N interference-reduced signals comprising a subset of the K separate signals.
 14. The system of claim 12, wherein said one or more circuits: despreads of the first set of N channel signals so as to generate a set of N despread baseband signals; synthesizes an interference signal as a function of the set of N despread baseband signals; and subtracts the interference signal from the N despread baseband signals, and thereby removes interference due to the first set of N channel signals from the second set of M channel signals.
 15. The system of claim 14, wherein said one or more circuits comprises N threshold detectors, and wherein said one or more circuits: passes each of the N despread baseband signals through a corresponding one of said N threshold detectors so as to generate an estimated set of N symbol values for the first set of N channel signals; spreads each of the N symbol values according to a corresponding one of the N orthogonal sequences so as to generate a first baseband composite signal; and scrambles the first baseband composite signal according to the first scrambling sequence so as to synthesize the interference signal.
 16. The system of claim 13, wherein said one or more circuits: despreads the first set of N channel signals so as to generate a set of N despread baseband signals; despreads the second set of M channels signals so as to generate a set of M despread baseband signals; and subtracts, from each of the N despread baseband signals, an interference signal synthesized as a function of the M despread baseband signals thereby removing interference due to the second set of M channel signals from the first set of N channel signals.
 17. The system of claim 16, wherein said one or more circuits synthesizes the interference signal as a function of estimated symbol values generated from the M despread baseband signals.
 18. The system of claim 10, wherein the signal complies with a communication protocol selected from the group consisting of: orthogonal frequency division multiplexing (OFDM), time division multiple access (TDMA), code division multiple access (CDMA), gaussian minimum shift keying (GMSK), complementary code keying (CCK), quadrature phase shift keying (QPSK), frequency shift keying (FSK), phase shift keying (PSK), and quadrature amplitude modulation (QAM). 